A variety of integrated circuit devices require the generation of a stable bias current by a portion of the device. The bias current is used to set the magnitude of the currents used to power the various components of the device. It is very important that the bias current remain as near as possible to a predetermined level to insure that the total current required by the integrated circuit device is constant and therefore predictable.
A variety of forces acting upon the integrated circuit device can create fluctuations in the bias current level. The two most significant forces are the ambient temperature in which the device is operating, which causes bias current fluctuations, and the variations which are introduced in the device during the construction of the device, which affect the magnitude of the bias current.
A typical example of an integrated circuit device which needs a stable bias current supply is a low-power operational amplifier. Because these amplifiers are used in low-power situations, these devices require relatively simple bias circuits with few components and minimal current requirements. Bias current supply circuits for high precision operational amplifiers tend to include a large number of components in order to provide a stable output current. However, these circuits require more current than would be acceptable for a low-power design.
Simpler bias current supply circuits in use with low-power designs consume less current, but they are more susceptible to process variations and ambient temperature changes. As a result, the performance of the low-power operation amplifier suffers. Hence, bias current supply circuits presently in use meet the requirement of simple design with low current requirements. While these circuits normally include rudimentary temperature compensation through the use of conventional resistors, they provide no protection against process variations. Additionally, due to the fact that process variations in resistors are particularly difficult to control, the addition of these resistors to conventional bias current supply circuits augments the problem of process variations.
Therefore, a need has arisen for a simple bias current supply circuit using a relatively small number of components and having correspondingly small current requirements which can provide an output current that is stable with respect to both ambient temperature variations and process variations.